The present invention is in the veterinary field. More specifically, the invention relates to the prophylaxis and therapy of pleuropneumonia in pigs.
Pleuropneumonia is a major respiratory disease in pigs and causes severe economic losses in pig farming in many countries including the United States and Canada. The disease is caused by the bacterium Actinobacillus pleuropneumoniae (previously also referred to as Haemophilus pleuropneumoniae) and is considered to be one of the most important disorders of the bronchial tubes in pigs. Frequently, the disease is fatal. Actinobacillus pleuropneumoniae is known to exist in twelve infective serotypes.
Since pleuropneumonia can be induced by inoculating pigs with sterile culture supernatants of A. pleuropneumoniae, extracellular toxic proteins are assumed to be involved in the development of the pneumonic lesions. There is growing evidence that qualitative or quantitative differences in toxic activities exist between the twelve serotypes of A. pleuropreumoniae. Hemolytic and cytotoxic activities have been reviewed by T. A. Bertam, Can. J. Vet. Res. 54: S53-S56 (1990). Two different hemolytic activities were reported by Frey and Nicolet, J. Clin. Microbiol. 28: 232-236 (1990), whereas four antigenically different activities were distinguished by Kamp and Van Leengoed, J. Clin. Microbiol. 27: 1187-1191 (1989). Whether such activities are functions of one or more molecules is not known.
Vaccines proposed thus far for preventing infections by Actinobacillus pleuropneumoniae are mostly based on whole live cells, attenuated cells, lysates, culture supernatants, or extracts of A. pleuropneumoniae. WO-A-80,02113 (or Canadian Patent 1,189,790) teaches a vaccine for controlling pleuropneumonia in pigs, containing A. pleuropneumoniae cells, cell fragments etc. and, as an adjuvant, material derived from Bordetella pertussis. EP-A-420,743 proposes a vaccine containing inactivated toxin of serotype 1 and optionally an inactivated toxin of another serotype of A. pleuropneumoniae; it provides protection against serotype 1 and partial protection against other serotypes. EP-A-354,628 discloses a universal vaccine against A. pleuropneumoniae, which contains extracellular proteins from two different serotypes, and is effective against all A. pleuropneumoniae serotypes. Although these known vaccines provide protection against some or even all of the field strains of A. pleuropneumoniae, the active compounds are not known. As a result, control, verification, and standardisation of vaccines is difficult, since the ratio between active components cannot be optimized and inactive and sometimes adverse components are always present in the vaccines.
The present invention, in one aspect, provides a vaccine for the prevention and/or the treatment of infection by Actinobacillus pleuropneumoniae containing at least an immunogenic part of at least one polypeptide selected from the group consisting of cytolytic proteins of A. pleuropneumoniae produced by recombinant DNA technology, and detoxified derivatives thereof.
It has been found according to the invention that Actinobacillus pleuropneumoniae produces three hemolytic and/or cytotoxic proteins (toxins), hereinafter referred to as cytolytic proteins: Cytolysins I, II and III (ClyI, ClyII and ClyIII). Where the term xe2x80x9cCytolysinxe2x80x9d (Cly) is used in the present specification, this shall thus be understood to comprise any extracellular protein produced by any strain of A. pleuropneumoniae and producing any adverse effect (be it hemolytic, cytotoxic or other or both) on cells or tissues of an infected animal; where appropriate it shall be understood also to comprise immunogenically active parts of these proteins or derivatives thereof having diminished adverse effects. Protection against infections by any of the known serotypes of A. pleuropneumoniae is conferred to an animal by administering an effective amount of all three cytolysins, and partial or complete protection against specific serotypes is conferred by administering one or two of the cytolysins, depending on the serotype or serotypes in question.
Thus, the vaccine of the invention contains at least one of the three cytolysins I, II and III, preferably two, and more preferably three. The cytolysins may be present in the vaccine as the naturally occurring proteins, or they may be present as derivatives containing at least an immunogenic part of the proteins, or as a detoxified equivalent. Detoxification shall be understood to mean that the toxic activity of the proteins has been removed to a sufficient degree or for a sufficient number of the protein molecules to provide a vaccine which does not produce an unacceptable toxic reaction in the producing host and/or in the vaccinated animal, whereas it provides a sufficient immune response. Detoxification can be brought about by chemical, physical or enzymatic treatment of the proteins or by substitution, insertion or deletion of one or more nucleotides in the cytolysin genes resulting in the substitution, insertion or deletion of one or more amino acids in the protein. Detoxification can also be achieved by expression of the toxin gene in the absence of the activator gene.
It was found that the cytolysins are encoded by operons wherein the structural toxin gene is flanked at the 5xe2x80x2 end by a gene encoding a peptide required to activate the toxin, hereinafter referred to as the activator protein. The cytolysins may be present in the vaccine in the activated or non-activated form.
The cytolysins or their derivatives present in the vaccine are preferably obtained by expression of recombinant DNA encoding the proteins mentioned above. The detoxified cytolysins constitute a further embodiment of the present invention.
In another aspect of the invention a process for producing a cytolytic protein of Actinobacillus pleuropneumoniae or an immunogenic and/or detoxified derivative thereof is provided, which process comprises the steps of:
a) selecting at least one nucleotide sequence coding for at least an immunogenic part of said cytolytic protein (toxin) optionally including an activator protein, or a derivative thereof;
b) inserting the nucleotide sequence(s) selected in step a) in a vector or an expression vector;
c) transforming a host cell, preferably a host cell that is capable of secreting said cytolytic protein, with the vector obtained in step b);
d) cultivating the host cell of step c) to express the nucleotide sequence(s) of step a),
e) recovering and optionally purifying the protein from the culture;
f) optionally modifying the protein to produce a detoxified protein.
In yet another aspect, the invention is concerned with a process of producing a vaccine wherein at least one, and preferably two, and more preferably three, of the cytolysins or immunogenic parts thereof, thus produced, are combined with an immunologically acceptable carrier and optionally a suitable adjuvant.
The host cell referred to in the process of producing the cytolysins or their derivatives may be a microorganism, preferably a non-pathogenic microorganism capable of expressing at least one nucleotide sequence encoding the cytolysins by having a strong promoter inducing high expression levels or by allowing the introduction of an exogenous promoter system to induce such high expression levels. A suitable host cell is Escherichia coli. 
In a further aspect, the invention provides a nucleotide sequence encoding at least an immunogenic part of a polypeptide selected from cytolytic proteins of Actinobacillus pleuropneumoniae optionally including activator proteins and transport proteins, the latter ones being proteins that assist in the secretion of the cytolytic proteins to the periplasma or the medium. The invention also relates to a system that expresses and secretes said nucleotide sequence and to a vector containing at least one of said nucleotide sequences each one preferably operatively linked to a promoter and optionally an enhancer.
In yet another aspect the invention relates to a host cell containing at least one nucleotide sequence encoding the cytolytic proteins or their derivatives, and capable of expressing them, the nucleotide sequence(s) either being contained as such or as said vector and being either present in the host cell in the genome of the host or as a plasmid. Preferably, the host cell contains nucleotide sequences encoding at least two of the cytolysins, and more preferably it contains the sequences encoding all three cytolysins. The host cell is preferably derived from E. coli. 
The invention also provides a vaccine for prophylaxis and therapy of infections by A. pleuropneumoniae containing a microorganism carrying one or more nucleotide sequences encoding at least an immunogenic part of at least one cytolytic proteins of A. pleuropneumoniae or a detoxified derivative thereof. The microorganism may be an attenuated microorganism such as an attenuated virus or a bacterium. Administration of the vaccine results in multiplication of the microorganism and thus in production of the immunogen.
The invention further relates to diagnostic means for detecting infection by A. pleuropneumoniae. Specifically, the invention is concerned with an antibody, preferably a monoclonal antibody, raised against one of the native cytolysins and useful as a component of a diagnostic kit for detecting infection by A. pleuropneumoniae; antibodies raised against modified cytolysins are useful for determining protection by these modified cotylysins. Antibodies raised against native or modified cytolysins can also be used for passive immunisation of infected animals.
In another aspect, the invention provides a DNA-probe comprising at least a part of a nucleotide sequence encoding a cytolysin of Actinobacillus pleuropneumoniae which may be used in a diagnostic method and a diagnostic kit for detecting infection by A. pleuropneumoniae. Another method of diagnosing an A. pleuropneumoniae infection is to determine the presence of A. pleuropneumoniae cytolysins in a subject whereby protein pattern is indicative of the infective serotype or group of serotypes.